When installing FreeBSD from an ftp/http it may complain that MANIFEST file is missing, to solve the problem try this:

export DISTRIBUTIONS="kernel.txz base.txz"
mkdir /usr/freebsd-dist
export BSDINSTALL_DISTDIR="/usr/freebsd-dist/"
export BSDINSTALL_DISTSITE="https://download.freebsd.org/ftp/releases/amd64/amd64/12.1-RELEASE/"
bsdinstall distfetch

cd /usr/freebsd-dist
fetch https://download.freebsd.org/ftp/releases/amd64/amd64/12.1-RELEASE/MANIFEST

Then run bsdinstall